They are in Acmetropolis
Ace: Is this... Is this my home?
They went to the Secret Place
Mephiles: This world has been connected.
They saw the City disappeared
Wile: What was that?!
Mephiles: Tied to the darkness... Soon to be completely eclipsed, There is so very much to learn, You understand so little, A meaningless effort, One who knows nothing can understand nothing.
They saw Mephiles as Duck
Mephiles: Take a look at this tiny place, To the heart seeking freedom this city is a prison surrounded by water. And so this boy sought out to escape from his prison, He sought a way to cross over into other worlds. And he opened his heart to darkness.
Duck has turned into Mephiles
Mephiles: Don’t bother. Your voice can no longer reach him where he is. His heart belongs again to darkness. All worlds begin in darkness, and all so end. The heart is no different, Darkness sprouts within it, it grows, consumes it. Such is its nature. In the end, every heart returns to the darkness whence it came.
Ace jump back
Mephiles: You see, darkness is the heart’s true essence.
Ace: That’s not true! The heart may be weak. And sometimes it may even give in. But I’ve learned that deep down, there’s a light that never goes out!
Mephiles rised up
Mephiles: So, you have come this far and still you understand nothing. Every light must fade, every heart return to darkness!
He Summon his guardian and they fight him after that he's retreating from them and the City is falling apart. Ace went to stop Mephiles and the his Friends has been block by a Force Field, So Ace have to fight Mephiles on his Own after that Mephiles has been defeated